The present invention relates in general to semiconductor devices and, more particularly, to power supplies that include charge pumps formed on integrated circuits.
Power supplies used in electronic systems typically are designed to meet particular requirements of the systems. For example, power supplies used to power smart card readers provide supply voltages that may have a value of either three volts or five volts, depending on the specification of the smart card which is inserted in the card reader. Smart cards contains embedded integrated circuits that store users"" personal data as well as software programs for running smart card applications. Many card readers operate from a three volt battery for portability.
Before processing smart card data, a smart card reader determines which type of smart card has been inserted and then provides the corresponding supply voltage at a supply terminal. A three volt supply typically is provided with a transistor switch that connects the battery to the supply terminal. A five volt supply typically is generated with a charge pump that pumps charge from the battery to produce a five volt pump voltage at the supply terminal.
Card reader power supplies have a drawback in that their integrated circuits are susceptible to a destructive latchup condition caused when a transistor electrode forward biases with respect to the well region in which the transistor is formed to turn on a parasitic transistor. Previous power supplies bias the well regions to the battery voltage to maintain a reverse bias on the well regions. However, when the pump voltage is greater than the battery voltage, a latchup may still occur. If the well regions are biased to the pump voltage, a latchup can occur when the charge pump is disabled or while the charge pump is starting up, when the battery voltage is greater than the pump voltage.
Hence, there is a need for a power supply and method of providing supply voltages which reduces the susceptibility to a latchup condition in the power supply""s integrated circuits in order to improve reliability and avoid damage to the power supply.